There are many impurities/contaminants present in the downstream hydrocarbon products like LPG, gasoline, naphtha, kerosene, diesel etc, from petroleum crude processing units. These contaminants appear in hydrocarbon products due to natural sources or are produced during passage through different processing stages. The impurities like napthenic acid in diesel, hydrogen sulfide, mercaptans and COS in gasoline, LPG, naphtha and kerosene obtained from different units are required to be removed to meet the desired specifications which are becoming more and more stringent in order to meet the environmental statutory regulation.
These hydrocarbons are treated with either caustic or amine solution, or by both, in a contactor. As will be apparent to a person skilled in the art, a contactor should ensure proper contacting of immiscible liquid—liquid systems. Such contactors could be a packed column, tray column, mixing valve or static mixer depending on specific application. For example, for removing H2S/mercaptan from a hydrocarbon stream, a mixing valve is used. In a mixing valve, one phase gets dispersed into the other phase as droplets, at the expense of shear energy. In this process, the ratio of hydrocarbon and caustic phase is important for maintaining efficiency of the process. Such process is effective when the reaction of the impurities with the second liquid is very fast, However, such processes suffer carryover of the dispersed phase along with the continuous phase. All these processes generate a certain degree of turbulence between the phases.
Contactors are also known in which minimum turbulence is generated in the contact zone, such contactors are made of fibre bundles (U.S. Pat. Nos. 3,754,377; 3,758,404; 3,839,487 and 5,904,849). In such contactors, a series of thin fibre strands are housed in a column. U.S. Pat. No. 3,758,404 describes one such arrangement and U.S. Pat. No. 3,992,156 describes an improved contactor.
For certain applications, for e.g. removal of napthenic acid from diesel, the above contactors other than fibre film contactors can not be used, as turbulence of phases creates an emulsion which is stabilized by the sodium naphthenic, which acts like a surfactant. Sodium naphthenate is formed during reaction of caustic with naphthenic acid. Conventional contactors, e.g. mixing valves, static mixers do not work due to emulsion formation. Even with fibre film contactors, using a conventional distribution system, emulsification could not be avoided. Therefore, there is an urgent need for developing a novel distribution system to ensure that turbulence in the interface of caustic and hydrocarbons is avoided.
Napthenic acids are the main contributors to acidity of the diesel. The napthenic acids react with caustic according to the following equation.R—COOH+NaOH=R—COONa+H2O
The sulfur compounds such as hydrogen sulfide and mercaptans react with caustic according to the equations as follows:H2S+NaOH=Na2S+2H2ORSH+NaOH=RSNa+2H2O
Depletion rate of impurities is controlled by mass transfer rate as reaction rate is very high.
The mass transfer rate of reacting species can be expressed as:R=(k)(AXAC)Where
k mass transfer coefficient,
A Interfacial area,
AC concentration gradient
In conventional treating mechanisms, devices such as mixing valves and static mixers create an interfacial area by dispersive mixing, which generate droplets. To maximize the surface area for a given volume, considerable shear energy must be imparted to form as many small droplets as possible. Also droplet size has to be as small as possible. Small droplets, however, have the disadvantage of taking a longer time to separate from the bulk phase. Consequently, a large separator is provided.
The Continuous Film Contactor (CFC) is a static contacting device that produces non dispersive contacting of the caustic and hydrocarbon phases and improves the removal of acidic/sulphidic impurities from hydrocarbon streams. This prevents emulsion formation and results in minimum caustic carry over and high utilization of caustic solution.
The contactor, containing fibers, provides a large interfacial area, which increases the mass transfer rate. At the same time, the aqueous phase is constrained to the fibre material by surface tension forming a film on each fibre that contacts, but never mixes with the hydrocarbon phase. Consequently, separation of phases becomes a simple and efficient step in the process.
The efficiency of the mass transfer of undesirable contaminants from hydrocarbon phase to aqueous phase depends on the distribution of both of the liquid phases into the fibre packing. Also, packing of fibers and the fibre holding arrangement play an important role. Packing of fibers should be in such a way that it is equally and evenly distributed across the cross section and throughout the column length, so that caustic and hydrocarbon in any portion of the column should not pass without seeing each other. The interfacial area of mass transfer depends upon how the column is packed i.e., fibre diameter, number of fibers.